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Devit theory of old glass objects
Posted: Sun Sep 14, 2003 2:26 am
by Lauri Levanto
Trying to understand more how glass behaves
I have noticed that _very_ old bottles and glasses
have a grayish matte surface.
I suppose that they were made clear.
1. Is the matte due to devit - that is does glass devit
slowly in room temperature?
2. Is the matte due to mechanical wear and tear?
It looks too even for miscellaneous scratches.
3. Does some substances, maybe fluxes or sodium
chemically dissolve in unfavourable circumstances?
If so, is the use of overspray detrimental?
-lauri
Posted: Sun Sep 14, 2003 7:02 pm
by jerry flanary
The surface of old glass usually changes over time if exposed to water. I don't quite understand it all yet but the flux gets leached out. This is why old dug up glass loooks iridescent or why some even turns pinky-purple. I know the pinky purple stuff is from using Manganese Dioxide to make the glass more "clear" in appearance. The purpleness counters some yellowness in the glass so it looks clear. But then over time whatever caused the yellowness leaches out and the purple looks strong.
Old glass is nicer just being old don't you think?
Well if you are going to mess with it I would flux it. But I don't know anything I'm just talking like a drunk.
j.
Posted: Mon Sep 15, 2003 8:11 am
by Lauri Levanto
Thanks Jerry,
I'm NOT going to spoil old glass,
only trying understand what happens when I
leave an object out of hand.
-lauri
Posted: Mon Sep 15, 2003 1:11 pm
by charlie
jerry flanary wrote:The surface of old glass usually changes over time if exposed to water. I don't quite understand it all yet but the flux gets leached out. This is why old dug up glass loooks iridescent or why some even turns pinky-purple. I know the pinky purple stuff is from using Manganese Dioxide to make the glass more "clear" in appearance. The purpleness counters some yellowness in the glass so it looks clear. But then over time whatever caused the yellowness leaches out and the purple looks strong.
Old glass is nicer just being old don't you think?
Well if you are going to mess with it I would flux it. But I don't know anything I'm just talking like a drunk.
j.
uv light over time turns the colors. examples are lots of old bottles found in the desert.
the surface could be etched from the acid rain or hard water deposits, causing a milky affect.
Re: Devit theory of old glass objects
Posted: Tue Sep 16, 2003 3:17 am
by Jerry Barnett
lauri wrote:Trying to understand more how glass behaves
I have noticed that _very_ old bottles and glasses
have a grayish matte surface.
I suppose that they were made clear.
1. Is the matte due to devit - that is does glass devit
slowly in room temperature?
From my limited understanding of glass chemistry, I think devitrification is impossible at room temperatures regardless of the length of time. The spatially random linkage of molecules in the glass state would have to be broken to be able to form the repeated hexagonal links of the crystal form. The energy supplied by room temperature would be a fraction of what would be required.
2. Is the matte due to mechanical wear and tear?
It looks too even for miscellaneous scratches.
Maybe, but I think 3 is more likely.
3. Does some substances, maybe fluxes or sodium
chemically dissolve in unfavourable circumstances?
Water infiltrates the glass surface and takes sodium ions into solution leaving a surface that has low alkali content. Initially, this layer is very thin, 1/1000 of a mm. If it grows thicker, it becomes visible as dullness. Container glass, designed for manufacturing ease instead of durability, has a high level of NaO flux and is particularly susceptible. Art glass, I suspect, has a higher concentration of CaO - which inhibits this condition - than does container glass.
If so, is the use of overspray detrimental?
Good question. I would guess they are not. Lead and boron oxides increase chemical resistance of glass. Could be more of a question with Super Spray if the glass used has a high NaO concentration so that it flows readily, but the temperature required to actually mature Super Spray would suggest that the concentration is not that high.
-lauri